Method of bonding aluminum to steel



Jan. 23, l1951 F. R. HENsEL 2,539,247

METHOD oF BONDING ALUMINUM To STEEL Filed July 51, 1945 3i 'Il *2 h, w, u *c z LL Ll` QT u: V) g b k gk N Q m EQ: k wak v: uw u l v: m u V: Q

v lNvENTOR N /r franz E fia/n.5?!

ATTORNEY Patepnted Jan. 23, 1951 METHOD OF BONDING ALUMINUM TO STEEL Franz R.' Hensel, Indianapolis, Ind., assignor to P. R. Mallory & Co., Inc., Indianapolis, Ind., a

corporation of Delaware Application July 31, 1945, Serial No. 607,993

4 Claims.

This invention relates to the art of metal bonding, and, more particularly, to a novel metlfod of permanently bonding layers of similar or dissimilar metals together, and to bimetals or overlay metals produced by such method.

The present application is a continuation-inpart of my co-pending applications Serial No. 557,703, filed October 7, 1944, and Serial No. 579,992, filed February 27, 1945, both of said aD- plications being entitled Aluminum Clad Steel.

In the above-identified co-pending applications, I have disclosed that a strong, ductile and fatigueresistant bond may be obtained between a layer of iron group metal and a layer of aluminum base metal by providing a bonding layer of silver between the two metals. One method of making overlay metals of the described character comprises bonding a layer of silver to the surface of the iron group metal and another layer of silver to the surface of the aluminum base metal and then bonding the layers together by heat and pressure. The preferred method of applying the silver to the iron group metal and to the aluminum base metal is electroplating, although other methods may also be employed such as silver vapor deposition, pressure bonding, or fusion bonding. The clean silver-coated surfaces of the iron group metal and of the aluminum base metal are then placed in contact and heated in a nonoxidizing atmosphere to a temperature which may be in the range of 350 to 500 C. and the two sheets are then pressed between pressure rolls, platens, or other pressure applying means to complete the bond. As it is further disclosed particularly in the said co-pending application Serial No. 579,992, the provision of a silver layer on the surface of the aluminum base metal may be dispensed with and a layer of silver may be provided only onfthe surface of the iron base metal. Thus, for example, a thin layer of si'ver may be produced on a steel surface by electrodeposition. It is essential that the silver adhere tightly to the steel, therefore, the surface of the steel must be carefully prepared to receive the silver layer by cleaning, sand blasting, pickling and bright annealing, if necessary. The aluminum, or aluminum base metal must also be carefully cleaned and prepared in order to present a clean and bright metallic surface. In general, the type of surface cleaning usually given to aluminum prior f to resistance welding is adequate. The clean silver-coated surface of the steel is now placed in contact with the clean but uncoated surface of the aluminum and both surfaces are heated in a non-oxidizing atmosphere to a temperatulfl? 2 in the range of 350 to 525 C. and the two sheets are bonded together by the application of pressure thereto.

I have now discovered that there is a definite inter-relation between the thickness of the silver interlayer and the strength of the bond obtained. More particularly, it has been found, contrary to all expectations, that the strength properties of the bond are improved by reducing the thickness of the silver interlayer. While silver layers having a thickness between 0.0003 and 0.006" provide generally satisfactory bonds, the properties of the bond, such as its strength and fatigue resistance, can be further improved to a very substantial extent by applying a silver layer to at least one of the metals to be bonded, having a thickness of about 0.000025 to about 0.001", the preferred thickness being in the range of about 0.0001. Experience has demonstrated that when employing a relatively thick silver interlayer, the strength of the bond was substantially that of silver, which is a comparatively soft and weak metal, particularly at slightly elevated temperatures. The bond provided by such relatively thick silver interlayers, if it was subjected to heating cycles at temperatures of about 200 C., would frequently deteriorate. This was a disadvantage for certain practical applications of the bimetals made in accordance with the invention, such as for bearing applications, where the operating temperature may be as high as 150 C., or over. It was found that by controlling the thickness of the Silver layer within rather close limits, the permanency and the strength of the bond Were retained even though the bimetal was exposed to elevated temperatures for long periods of time. A microscopical investigation indicates that during the bonding process several aluminum-silver alloys are being formed, of which in some cases as many as three distinctive layers were identified. In the presence of an excess of silver, it is possible that all stable phases, such as alpha, gamma and delta may coexist. In heating further diffusion occurs and certain silver-aluminum compounds may be formed which are mechanically weak. By eliminating the excess silver, this danger is eliminated, as was demonstrated by actual tests. In addition to increasing the strength of the bond, reduction in the thickness of the silver interlayer has the further important advantage that it results in considerable economy in the use of silver.

Accordingly, it is one of the objects of the invention to provide a novel and improved method of bonding layers of aluminum and steel together by means of a silver interlayer and by the application of heat and pressure to the bonding region.

It is another object of the present invention to provide an improved method of bonding together layers of aluminum and steel by applying a silver' coating of controlled thickness to the bonding surface of the steel layer, bringing said silver coating into face to face contact with the bonding surface of the aluminum layer, and effecting a bond therebetween by pressure applied thereto concurrently with heating to suit'- able temperatures.

Other and further objects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawing, which is a diagrammatic illustration of a process for making bimetals or overlay metals in accordance with the invention.

Broadly stated, the bonding process embodying the present invention essentially comprises cleaning the surfaces of the strips of aluminum and steel to be bonded together by means of a mechanical, chemical or electrical procedure. A silver layer of critically controlled thickness is deposited upon the bonding surface of the steel strip either directly or on a strike rst provided thereon, such as a nickel or copper strike. The strips to be bonded together are now heated to appropriate bonding temperatures in a suitable inert or reducing atmosphere. Thereafter, the surfaces of the strips are pressed together to effect the bond, such pressure, being preferably produced by means of a pass between a pair of rolls, whereby at the same time a reduction in overall thickness of the bimetal is obtained. Of course, great variations are possible in carrying out the treatments for cleaning, plating, heating and pressing together the strips, such treatments being to a substantial extent determined by the characteristics of the individual metals to be bonded.

In order to give. to those skilled in the art a better understanding of the invention. the following illustrative example may be given:

EXAMPLE Bonding aluminum and aluminum base alloys to steel and other iron base metals A suitable method for producing aluminum clad steel with a cladding on one side nr both sides of the steel may comprise the following operations:

The steel is degreased with an organic solvent and is anodically cleaned with a suitable cleaning medium, such as Anodexf This is followed by rinsing the steel with water and pickling the same anodically in a pickling solution. which may be a solution of orthophosphoric acid in water, with a current density of about '70 to 100 amperes per square foot. After the pickling operation, the steel is again rinsed with deionized water and is transferred into a nickel strike bath for applying a nickel strike thereto. A suitable nickel strike solution may consist of the following ingredients:

Grams per liter NiSo4f7H2O 202 to 210 NHiCl 33.8 to 48 H3B03 26 130' 37 Dupenol NE, dry 0.05 to 2 The pH of the nickel strike solution is held between 4.6 and 5.2, and the strike may be applied for a period of about three minutes at a current density of about 10 amperes per square foot.

After the nickel strike, the steel Yis again rinsed with water and after a dip in KCN a silver strike is applied thereto in a bath of silver cyanide and potassium cyanide solution. Immediately after the silver strike has been applied, the pretreated steel is immersed in the silver plating bath. A suitable silver plating bath may contain the following ingredients:

Grams per liter A small percentage of a brightener may be added to this bath. The steel is silver-plated for approximately fty seconds at 45 amperes per square foot and at a temperature of F. Such a plating procedure will produce a silver deposit of a thickness of about 0.0001", which is a preferred silver thickness for the purposes of the present invention.

The aluminum surface may be prepared for the bonding process in the following manner:

The aluminum is rst thoroughly degreased with carbon tetrachloride or withsome other similar organic solvent, and is cleaned in a special aluminum cleaning solution, such as Enthon E. The aluminum is then rinsed in water and dipped in a solution of nitric acid Yand is again rinsed and dried.

The aluminum and steel are then held together by any suitable means, with the silver layer on the steel facing the cleaned surface of the aluminum, and are heated in a non-oxidizing atmosphere or in a reducing atmosphere, such as hydrogen. cracked gas, or cracked ammonia. to a temperature between 350 and 525 C., for example to about 500 C. If the bonding process is a continuous one, heating facilities have to be provided which are capable of raising the temperature of the assembly to the desired bonding temperature at a sneed corresponding to the speed of the rolling operation. This may be accomplished by induction heating. torch heating, resistance heating or any other suitable means. If the bonding process is of the batch type, the heated assembly of aluminum on steel may be removed from the furnace and may be passed through a rolling mill effecting a total reduction of 10# to 40%. While in general it is not desirable to expose the heated assembly to air, experiments have shown that short exposures are not harmful. It has been also found that a silver layer of about 0.0001" in thickness is suficient to protect the steel from oxidizing, when exposed to air for short periods of time.

When it is desired to manufacture aluminumsteel overlay metal on a quantity production scale, it is preferred to carry out the cleaning, plating, preheating and rolling operations continuously'. Such a continuous process of making aluminum-steel overlay metal is diagrammatically illustrated in the accompanying drawing.

A strip of steel sheet I0 is unwound from roll Il and passes through a cleaning and electroplatin-g apparatus I2 which applies a layer of silver to one surface of the steel strip. An aluminum strip I3 is simultaneously unwound from roll I4 and passes through a cleaning apparatus l5 to clean and to prepare at least one of its surfaces. The silver-plated steel strip I and the cleaned aluminum strip I3 are then brought together with the silver-plated face of the steel in contact with the cleaned face of the aluminum and led through a hydrogen furnace I6 which heats them to pressure bonding temperature in a hydrogen atmosphere. The contiguous strips emerge from the furnace through a hydrogen chamber II and then pass between pressure rolls I8 and I9 which apply sufficient pressure to bond the silver surface of the steel together with the cleaned surface of the aluminum, thus completing the bimetal strip' 20 which emerges from the process.

The rolling process may be controlled in such a manner as to provide the desired ratio of thickness between aluminum and steel after a single pass through the mill. If the rolls are kept in good condition, an excellent surface finish may be obtained both cn the steel and on the aluminum. In certain cases, however, it may be desirable to use a breakdown mill exclusively for the bonding process and to subject the bimetal to additional cold rolling operations in 2 high, 4 high or cluster mills until it is reduced to the desired dimensions. Experiments have shown that cold reductions exceeding 60% may be applied without affecting the strength of the bond.

As a result of its comparative softness, aluminum will be reduced at a rate higher than steel. Some of the factors affecting the aluminum to steel ratio are the bonding temperature, the friction between rolls and work, the hardness of components at the rolling temperature, the speed of rolling, the original starting ratio, and the total reduction. The ratio, of course, may be readily adjusted to any desired value by appropriate control of any or all of these factors.

Applications of aluminum clad steel are in industrial fields where it is desired to combine the properties of aluminum, such as light Weight, good electrical and thermal conductivity, high resistance to corrosion, good frictional characteristics, with the high tensile strength of backing members, such as steel, or other ferrous alloys.

Specific applications of aluminum clad steel prepared in accordance with the principles of the present invention are,ffor example, bearings, containers for the dairy and chemical industries, light-weight component parts for electrical and mechanical devices, such as steel springs clad with aluminum, and the like. Stainless steelaluminum overlay metals may be advantageously employed in the manufacture of cooking utensils wherein the aluminum may be clad on one or both sides of the stainless steel. Cooking utensils of this type are greatly superior to conventional utensils of the type wherein copper is clad with stainless steel in that aluminum provides the advantage of light weight in addition to excellent heat conductivity. As the operating temperatures of cooking utensils are relatively high, it is of critical importance to provide silver interlayers of greatly reduced thickness in accordance with the present invention as the bond strength at such temperatures would be greatly reduced by utilizing heavy interlayers.

Heretofore, great difliculties have been experienced in resistance welding aluminum and aluminum alloys, due to the high electrical and thermal conductivity of such materials. The useful life of welding electrodes or tips Was rather short when welding aluminum and its alloys, and

'of said body, each constituting 10% of the total thickness. Overlay metals of this improved type combine the advantage of light weight and increased modulus of elasticity with great facility of welding and may be employed in the fabrication of complex structures by conventional resistance welding methods without the difficulties encountered in welding unclad aluminum strips.

What is claimed is:

l. The method of making aluminum-steel overlay metal from layers of aluminum and steel which comprises cleaning the bonding surfaces of the aluminum and steel layers, coating the bonding surface of the steel layer with a layer of silver having a thickness between about 0.000025 and about 0.001", placing said silver coating on the steel layer in contact with the cleaned surface of the aluminum layer, preheating the layersunder non-oxidizing conditions to a temperature below the melting point of silver and between about 350 and about 525 C., and applying rolling pressure to said preheated layers to reduce the thickness of the layers and to permanently bond the layers together.

2. The method of making aluminum-steel overlay metal from layers of aluminum and steel which comprises cleaning the bonding surfaces of the aluminum and steel layers, electrodepositing a coating of silver having a thickness between about 0.000025 and about 0.001" on the cleaned surface of the steel layer, bringing said layers together with the silver coating on the steel layer in contact with the cleaned surface of the aluminum layer, preheating the layers under non-oxidizing conditions to a temperature below the melting point of silver and between about 350 and about 525 C., and applying rolling pressure to said preheated layers to cause reduction thereof and to permanently bond the layers together.

3. The method of making aluminum-steel overlay metal from layers of aluminum and steel which comprises cleaning the bonding surfaces of the aluminum and steel layers, electrodepcsiting a coating of silver having a thickness in the order of about 0.0001 on the cleaned surface of the steel layer, bringing said layers together with the silver coating on the steel layer in contact with the cleaned surface of the aluminum layer, preheating the layers under nonoxidizing conditions to a temperature below the melting point of silver and between about 350 and about 5.? C., and applying rolling pressure to said preheated layers to cause reduction thereof and to permanently bond the layers together.

4. rlhe continuous method of making aluminum-steel overlay metal from layers of alu.- neinum and steel which comprises cleaning the bonding surfaces of the aluminum and steel layers, passing the said steel layer through a silver plating bath to electrodeposit on the bonding sur- REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 234,736 Wheeler et al. Nov. 23, 1880 757,582 Wachwitz Apr. 19, 1904 1,256,954 Travers Feb. 19, 1918 1,539,577 Kirschner May 26, 1925 2,094,483 Weder Sept. 2S, 1937 Number Name Date 2,171,040 Merritt Aug. 29, 1939 2,269,523 Deutsch Jan. 13, 1942 2,269,839 Young Jan. 13, 1942 2,277,023 Steiner Mar. 17, 1942 2,289,572 Underwood July 14, 1942 2,294,404 Hensel Sept. 1, 1942 2,317,510 Barklie Apr. 27, 1943 2,366,168 Bakarian Jan. 3, 1945 2,392,917 Guinee Jan. 15, 1946 2,473,712 Kinney June 21, 1949 2,484,118 Reynoids oct. 11, 1949 FOREIGN PATENTS Number Country Date 29,577 Great Britain 1897 26,375 Great Britain 1903 545,023 Great Britain May 7, 1942 OTHER REFERENCES P. 130, Aviation Pub., by Gardiner Pub. Co., Inc., N. Y., Jan. 17, 1927.

Silver Industry, pub. 1940, pp. 41, 42, 189 and 271. 

1. THE METHOD OF MAKING ALUMINUM-STEEL OVERLAY METAL FROM LAYERS OF ALUMINUM AND STEEL WHICH COMPRISES CLEANING THE BONDING SURFACES OF THE ALUMINUM AND STEEL LAYERS, COATING THE BONDING SURFACE OF THE STEEL LAYER WITH A LAYER OF SILVER HAVING A THICKNESS BETWEEN ABOUT 0.000025" AND ABOUT 0.001", PLACING SAID SILVER COATING ON THE STEEL LAYER IN CONTACT WITH THE CLEANED SURFACE OF THE ALUMINUM LAYER, PREHEATING THE LAYERS UNDER NON-OXIDIZING CONDITIONS TO A TEMPERATURE BELOW THE MELTING POINT OF SILVER AND BETWEEN ABOVE 350* AND ABOUT 525* C., AND APPLYING ROLLING PRESSURE TO SAID PREHEATED LAYERS TO REDUCE THE THICKNESS OF THE LAYERS AND TO PERMANENTLY BOND THE LAYERS TOGETHER. 